[clang][cli] Prevent double denormalization

If both flags created through BoolOption are CC1Option and the keypath has a non-default or non-implied value, the denormalizer gets called twice. If the denormalizer has the ability to generate both flags, we can end up generating the same flag twice.

Reviewed By: dexonsmith, Bigcheese

Differential Revision: https://reviews.llvm.org/D93094